1. Field of the Invention
The present invention relates to a mold used in a molding method for thermoplastic resins. In particular, the present invention relates to a mold for molding thermoplastic resins having a narrow molding temperature range.
2. Prior Art
Thermoplastic resins can be molded by injection molding or injection-compression molding to shape products. In either molding method, a molding cavity is formed between a pair of male and female molds, and a molten thermoplastic resin, which is filled in the molding cavity, is shaped in the form of a final product only by the injection of the molten resin into the cavity, or by the injection and compression of the molten resin in the cavity.
In conventional molding molds, the downstream end of a conduit for supplying a molten resin opens into the molding cavity. A certain length of a conduit for a molten resin including such an opened portion is usually constructed in a straight form, and the upstream part of the straight conduit portion is warmed with a heater.
On the other hand, the mouth at the downstream end of the opened portion is bored through a nozzle which is formed independently from the mold, so that the heat from such a heater is not transferred to the mouth. Since the mouth generally communicates with a molding cavity, a mass of a resin remaining in the mouth (surplus part) is integrated with the body of a molded product, and removed from the mold together with the body. Therefore, the mouth has a tapered shape widening towards a molding cavity to make the removal of the surplus part easy. However, when the surface of the mouth is incompletely finished, the surplus part bites and clogs the mouth. In some cases, a sink mark is formed on the surface of a molded product opposite to the mouth region.
Under such circumstances, Japanese Patent No. 1280794 discloses a mold in which a mouth has a tapered form widening towards the upstream of a proximate conduit, and a switching pin for opening and closing the mouth has a tapered head which is exposed to an opened edge facing a mold cavity. With such a mold, no surplus part is formed in the mouth, and thus the above problems caused by a surplus part can be avoided.
However, the mold disclosed in Japanese Patent No. 1280794 cannot avoid the lowering of a nozzle temperature, and thus, under certain molding conditions, a mass of a resin may clog the mouth. In such a case, a molded product cannot be removed from the mold.
In particular, a resin having a small difference between a no-flow temperature and a decomposition temperature, that is, a resin having a narrow molding temperature range, tends to cause such clogging of the mold.
One object of the present invention is to smoothen the flow of a molten resin in a specific length of a conduit for a resin melt just before entering a molding cavity formed by a molding mold comprising a pair of a male mold and a female mold, wherein a molten thermoplastic resin is supplied into the molding cavity through the conduit for a resin melt, and then shaping the supplied resin while shutting off the conduit for a resin melt with a pin for opening and closing the conduit for a resin melt, and to prevent the clogging of the conduit by the resin at the downstream end of the conduit for a resin melt.
According to the present invention, there is provided a mold for molding a thermoplastic resin comprising a pair of a male mold and a female mold in which a molten mass of a thermoplastic resin is supplied into a molding cavity formed by the male and female molds through a conduit for a resin melt, and after the completion of the supply of the resin, the supplied resin is shaped while shutting off the conduit for a resin melt with a pin for opening and closing the conduit for a resin melt, wherein said conduit for a resin melt comprises a through-mouth of a nozzle which is formed so that it is exposed to the molding cavity, and a warmed conduit which is connected with the upstream side of the through-mouth, and said mold has a freely movable pin for opening and closing the conduit for a resin melt, which closes the conduit for a resin melt when it is tightly inserted in the whole region of the through-mouth and opens the conduit for a resin melt when it recedes from the through-mouth, the first heater which heats the wall of the conduit for a resin melt at a temperature suitable for the flowing of the thermoplastic resin when it is supplied, and the second heater which heats the through-mouth at a temperature suitable for the flowing of the thermoplastic resin when it is supplied.
When a thermoplastic resin is molded with the mold of the present invention, any surplus part, that is formed in the conventional molding molds, is not formed, since the thermoplastic resin is molded in the molding cavity while the whole region of the through-mouth is closed with the pin for opening and closing a conduit for a resin melt. Furthermore, the certain length of the conduit for a resin melt just before entering a molding cavity is maintained at a temperature suitable for the flowing of the thermoplastic resin, since the through-mouth and the warmed conduit, which are successively provided just before the molding cavity, are both heated with the heaters at specific temperatures.
In addition, a warming temperature distribution can be easily maintained at a desired distribution, since the wall of the warmed conduit and the nozzle are independently warmed.
In particular, when the nozzle has a built-in temperature sensor, and the nozzle temperature is maintained at a desired temperature based on the sensed temperature, the effects of the present invention are exalted. The effects of the present invention are further exalted when the temperature of the warmed conduit is kept at a set temperature by detecting the temperature of the warmed conduit which is heated by the second heater.
In the first preferred embodiment of the present invention, the pin for opening and closing a conduit for a resin melt is in the form of a column from its proximal end to its tip end, and the front face of the tip end is exposed to the molding cavity of the mold, and the surface of at least one of the through-mouth and the pin is coated with a smooth plated layer.
In this embodiment, the pin is tightly inserted in the through-mouth in a sliding manner, and closes the entire region of the mouth. Thus, even the fitting of a parallel bore and a column can surely close the conduit for a resin melt.
Furthermore, the supply pressure of a resin melt can be decreased, since the molten resin supplied in the molding cavity is in contact with the smooth plated surface at the downstream end of the conduit for a resin melt. In particular, when a thermoplastic resin having a narrow molding temperature range is used, the molten resin heavily adheres to the through-mouth, the wall of the warmed conduit, or the pin for opening and closing a conduit for a resin melt, but the plated layer can suppress the adhesion of the molten resin. Accordingly, the work to replace resins can be made easy and shortened.
In the second preferred embodiment, the plated layer of the first preferred embodiment comprises a three-element alloy of nickel, phosphorus and boron. Since such a plated layer can be formed by electroless plating, the plated layer is certainly formed on the inner surface of the through-mouth. In particular, such a plated layer has high heat resistance and also wear resistance. Thus, the durability of the wall of the conduit for a resin melt in such a part can be improved.
In the third preferred embodiment, the mold of the present invention has the first cooling means, which is provided in a region surrounding the mouth of the nozzle, and partly cools the specified annular region of the mold wall exposed to the molding cavity. The first cooling means can avoid the adverse influences of the heat generated by the first heater on a molded product.
In the fourth preferred embodiment, the mold of the present invention has the built-in second cooling means which partly cools the specified region of the mold wall facing the mouth of the nozzle. The second cooling means can avoid the adverse effects of the heat generated by the first heater on a molded product like the first cooling means.
In the fifth preferred embodiment, an insulating space is provided around the outer peripheries of the lower part of the nozzle and a supply tube so that the heat from the first heater and/or the second heater is not directly conducted to a region surrounding the part to which the nozzle is attached.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating-preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
Throughout this specification and the claims which follow, unless the context requires otherwise, the word xe2x80x9ccomprisexe2x80x9d, and variations such as xe2x80x9ccomprisesxe2x80x9d and xe2x80x9ccomprisingxe2x80x9d, will be understood to imply the inclusion of a stated integer or steps or group of integers or steps but-not the exclusion of any other integer or step or group of integers or steps.